This invention relates to a cathode structure for a plasma addressed liquid crystal display panel.
U.S. Pat. No. 5,077,553 discloses apparatus for addressing data storage elements. A practical implementation of the apparatus shown in U.S. Pat. No. 5,077,553 is illustrated schematically in FIG. 2 of the accompanying drawings.
The display panel shown in FIG. 5 comprises, in sequence from below, a polarizer 2, a channel substrate 4, a cover sheet 6 (commonly known as a microsheet), a layer 10 of electro-optic material, an array of parallel transparent data drive electrodes (only one of which, designated 12, can be seen in the view shown in FIG. 2), an upper substrate 14 carrying the data drive electrodes, and an upper polarizer 16. The channel substrate 2 is typically made of glass and is formed with multiple parallel channels 20 in its upper main face. The channels 20, which are separated by ribs 22, are filled with an ionizable gas, such as helium. An anode 24 and a cathode 26 are provided in each of the channels 20. The channels 20 are orthogonal to the data drive electrodes and the region where a data drive electrode crosses a channel (when viewed perpendicularly to the panel) forms a discrete panel element 28. Each panel element can be considered to include elements of the layer 10 and the upper and lower polarizers 2 and 16. In the case of a color display panel, the panel elements include color filters (not shown) between the layer 10 and the upper substrate 14. The region of the upper surface of the display panel that bounds the panel element constitutes a single pixel 30 of the display panel.
The anodes in the several channels are held at ground potential. When a suitable negative voltage is applied to the cathode in one of the channels, the gas in that channel forms a plasma that provides a conductive path at the lower surface of the cover sheet 6. If the data drive electrode is at ground potential, there is no significant electric field in the volume element of electro-optic material and the panel element is considered to be off, whereas if the data drive electrode is at a substantially different potential from ground, there is a substantial electric field in that volume element of electro-optic material and the panel element is considered to be on.
It is conventional to assemble a display panel of the kind shown in FIG. 5 by forming a channel substrate assembly, including the channel substrate and the cover sheet, forming an upper substrate assembly, including the upper substrate, the data drive electrodes, and the layer of electro-optic material, and attaching the upper substrate assembly to the channel substrate assembly. In manufacture of the channel substrate assembly, the cover sheet is placed over the upper surface of the channel substrate and is frit sealed to the channel substrate around the periphery thereof.
It is known to fabricate the cathodes in a PALC display panel using a nickel or chromium base layer having a coating of a finely divided rare earth hexaboride, such as LaB.sub.6, dispersed in a glass matrix. The rare earth hexaboride is used as the coating material on the nickel or chromium base layer because it has a low work function so that it is an efficient emitter of electrons, and has a moderate heat of sublimation so that it is sputter resistant. However, there are some features of this cathode structure that might be considered less than optimal in some applications. For example, it is desirable that the coating material should have a fairly high resistivity so that an electric field that will force electrons to the surface of the cathode for emission into the plasma can be established in the coating material, but the conductivity of a rare earth hexaboride is rather high and consequently the field may be considered smaller than optimum. Further, when the cathode is formed, it may have asperities (convex regions in which the radius of curvature of the surface is or approaches zero, such as points, ridges or edges of the cathode), such that the electric field is non-uniform over the cathode and consequently the current density is non-uniform, leading to high temperatures and the possibility of arcing between the cathode and anode in a given channel.
In a helium-neon (HeNe) laser, the cathode is typically a tube of aluminum. In a HeNe laser that operates in the normal glow condition, there is a tendency for sputtering to take place at the cathode. It has been found that the problem of sputtering at the cathode of a HeNe laser can be ameliorated by anodically oxidizing the aluminum cathode.
A layer of aluminum oxide can be formed on a body of aluminum either thermally, by heating in an oxidizing atmosphere, or anodically, by establishing an oxidizing plasma between the aluminum body connected as anode and a cathode. Thermal aluminum oxide is a good insulator. When thermal oxide is formed on a body of aluminum, the thickness of the oxide layer is generally uniform and does not depend on the topography of the surface. When an anodic oxide layer is formed on a body of aluminum, the thickness of the layer depends on the topography of the surface, because the rate at which the anodic oxide is formed depends on the current density and the current density depends on the topography. The anodic oxide is formed preferentially at asperities of the aluminum body.